The present invention relates to electric storage batteries or cells and, in particular, to a new method and apparatus or means for preparing and installing a porous compressible material effective in attenuating an explosion of combustible gases which accummulate in the head space of storage batteries.
As is well-known in the art, most types of electric storage batteries generate combustible gases during operation, which gases are either vented from the battery container into the atmosphere or are recombined within the battery in secondary reactions with the active materials. However, even in battery constructions which are intended to provide for the internal recombination of combustible gases, there are certain circumstances, such as inadvertent or abusive overcharge, where the recombination mechanism is ineffective and significant volumes of combustible gases will be generated.
It is also well-known that the combustible gases within the head space of a battery may be accidentally ignited and result in explosion of the battery. For many years, effective and reliable means have been sought for preventing or minimizing explosions in batteries and the hazardous effects thereof.
The ignition of combustible gases within the head space of a battery can be caused by either an internal or external ignition source. Combustible gases which are generated within a battery, if not effectively recombined, will eventually create a high enough internal pressure so they must be vented to the atmosphere. The venting is typically accomplished through the use of a simple open vent slot, a flame arrestor or a one-way relief valve, sometimes referred to as a "burp" valve.
However, should an external source of ignition breach one of the protective devices or should an ignition occur within the container, the combustible gases in the head space may explode. The concentration of gases, typically a mixture of hydrogen and oxygen, and the relatively large volume of the head space can result in an explosion which will shatter the container, cover or other components. In addition, the explosion will also often carry with it the liquid acid or other hazardous electrolyte from within the container.
Thus, it is not surprising that materials and methods for suppressing or minimizing the effects of explosions within batteries have been long sought. It is, of course, axiomatic that elimination of the open head space or substantially filling the head space with a solid material would virtually eliminate the possibility of an explosion simply because the presence of combustible gases would be eliminated. However, neither alternative is acceptable. An open head space is necessary in virtually all secondary storage batteries. First of all, the head space accomodates certain battery components, such as plate straps, intercell connectors, or terminals. In addition, in batteries which utilize free liquid electrolyte, sometimes referred to as "flooded" systems, open head space is necessary to accommodate variations in the level of the electrolyte as the battery is cycled or to provide space for acid movement under extreme conditions of use, such as abusive overcharge.
For many years, it has been known to fill the head space in a battery or cell, either partially or totally, with a porous material to inhibit the explosion of gases within the head space and quench any flame which may be formed, while still allowing the movement of gases and electrolyte through the material. For example, U.S. Pat. No. 2,341,382 discloses partially filling the head space with a loosely packed material, such as crushed stone or glass, diatomaceous earth, or glass wool. The disclosure in that patent suggests that the loosely packed filled material will not prevent the explosion of gases entirely, but by dividing the head space chamber into many minute interconnected cells, a rapid total combustion of the gases is prevented and, instead, a series of weak and inconsequential minor explosions will occur until the flame is quenched. It is believed that the general theory set forth in that patent, sometimes called the "chain termination" theory, is essentially correct and valid for a large variety of porous filler materials. However, notwithstanding the soundness of the theory and the development in the ensuing years of many improved porous materials, particularly plastics, there has been no large scale or general implementation of the technology. Thus, there still exists in the battery industry today a serious need for a material and method of utilizing it which will effectively attenuate hazardous explosions, but will otherwise not be detrimental to safe and efficient operation of the battery.
There are a number of factors which are believed to have generally inhibited or prevented the practical and useful application of explosion suppression or attenuation technology in batteries. Broadly, these factors include the creation of other hazards and detrimental effects on battery performance. As the head space of a battery is filled with a porous material, there will be a decrease in the actual remaining void volume in the head space inversely proportional to the porosity or effective void volume of the filler material. In other words, the more solids present in the filler material, the greater will be the reduction in the total head space volume filled with such material. As indicated above and particularly in flooded batteries, the loss of actual open head space volume will lessen the space available for electrolyte movement or electrolyte level variations.
It is known that high rate charging or excessive over-charge can result in vigorous gassing in many types of batteries. If the gas bubbles formed in the electrolyte cannot find ready and fairly direct channels to the battery vent openings, electrolyte may be upwardly displaced and overflow through the battery vents. This condition is known as electrolyte "pumping" and the damaging and hazardous effects of a corrosive electrolyte flowing out of a battery are obvious.
In addition, if a relatively large volume of electrolyte is drawn from the cells through wicking by a porous material in the head space or if the porous material otherwise retains the electrolyte with which it comes into contact, insufficient electrolyte may remain in the cells for proper electrochemical reaction and operation of the battery. Also, any material to be used as an attenuation material in batteries must possess certain other critically necessary physical properties. Such materials must have adequate resilience to retain their shape and to readily fill sometimes irregular shape of battery head space.
A number of porous plastic materials have been used in fuel tanks or similar containers as a means for reducing the explosion hazards. Both fibrous and cellular plastics of various kinds are disclosed in the art. U.S. Pat. No. 3,561,639 discloses the use of a single block of open cell polyurethane foam to fill the interior of a fuel tank.
Bulked fibrous plastic materials of many types have also been proposed for use as a means of arresting flames and reducing explosion hazards in fuel tanks. The filamentary plastic materials proposed for such use include polyolefins, nylon, dacron, polyesters, acrylics, and polyurethanes, as well as others. The materials are typically bulked or textured to provide high porosity and void volume by any of many well-known methods such as twisting, looping, crimping, needle punching and so forth. Examples of various types of such materials are described in U.S. Pat. Nos. 3,650,431, 4,141,460, and 4,154,357.
In the commonly-owned, copending application of Binder et al, entitled "Battery Explosion Attenuation Material and Method", there is described an improved porous plastic material which has a unique bimodal pore distribution including a major proportion of the small pores most effective in explosion attenuation and a minor porportion of large pores which are required to accommodate gas and/or electrolyte movement within and through the head space during battery operation. The unique bimodal function may be provided by using a single porous material, properly prepared and installed, or by using a composite of two different porous materials. Furthermore either filamentary or open cellular materials may be used.
The explosion attenuation material is preferably installed in the open head space of a battery cell so that it is retained therein in a somewhat compressed state, e.g. about 20% compression. Maintaining some compression on the material provides two separate benefits. If the material is maintained in compression within the head space, its inherent resilience will tend to cause it to fill the entire open volume of the head space before it has reached its free, fully expanded state. This helps assure there will be no significant open volumes within the head space which would allow an explosion of more than minor and insignificant proportions to occur. In addition, it has been shown that certain materials maintained in compression attenuate explosions better (result in lower peak pressures) than the same material in a free, uncompressed state.
The utility of the materials which have been identified as effective in attenuating explosions can be substantially enhanced by providing means for facilitating their preparation and installation and assuring that optimum performance will be attained after installation. Because the manufacture of storage batteries is fairly highly automated, it would be desirable to provide attenuation material which also could be adapted to automated installation. However, the convenience of automated installation cannot detract from the performance of the attenuation material after installation, and, indeed, it would be most desirable if the method and apparatus used in the preparation and installation of the material actually enhanced its ultimate performance.